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I want to power a uC project (uC is a STM32, supposed to be powered with 3.3V) over a LiPo battery. Since LiPo's output ranges from 4.2 volts to 3.7 volts, I feel unsure whether to use a step-down or a step-up converter, i.e. step-down or a step-up.

In the data sheet of the TPS63031, I saw in one characteristic (at the first page) an example of 3.3 V out for 3.6 V in, but I couldn't understand if this step-up converter, would also 'step-down' the maximum 4.2 volts to 3.3 volts.

Maybe I just misunderstood how DC-DC converters can be operated, since I thought for up- or down-converters, you always have to be significantly below your target voltage.

Thank you all very much :)

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  • \$\begingroup\$ It is a buck-boost converter, that can both step up and down the input voltage. Unlike a linear regulator, which needs a higher input than the output and has much lower efficiency. But I would still recommend to regulate the output of the buck-boost to get more stable voltage. \$\endgroup\$ – Eugene Sh. Jul 25 '18 at 12:35
  • \$\begingroup\$ Agree with crj11 answer. If your current is low - especially if you can throttle the frequency of your STM32F - then a linear regulator is probably your best - and most efficient - option. Switching regulators are only more efficient when current and/or voltage drop across the regulator circuit are high. Look for a low-drop out regulator since your minimum voltage is 3.7V and you are going to 3.3V. \$\endgroup\$ – slightlynybbled Jul 25 '18 at 13:21
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    \$\begingroup\$ "LiPo's output ranges from 4.2 volts to 3.7 volts" It's not clear who decided that lower value. You can choose to limit yourself to that voltage range, but that's a choice; it's not a LiPo's full output voltage range, as specified on its datasheet (which some people might mistakenly believe, from the initial statement). Looking at a typical LiPo discharge curve, you'd only be able to use around half of the battery capacity, if you stay within those values. So while it simplifies the choice of regulator, it also reduces available battery capacity, while increasing total number of cycles. \$\endgroup\$ – SamGibson Jul 25 '18 at 16:35
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Since the battery voltage is always higher than the required voltage, you need a "buck" or voltage reducing regulator, assuming that you want to use a switching regulator. However, unless maximum efficiency is required, it would be much less complicated and costly to use just a 3.3V LDO (Low Dropout Regulator). These can only reduce voltage.

An LDO uses a series pass element such as a MOSFET or transistor to drop the right amount of voltage from the input to produce the required voltage at the output. The only real down side is that the dropped voltage is dissipated as heat, with the total power of the voltage drop times the current. Since you have a relatively small drop and probably relatively low current, an LDO would not waste that much power.

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